1. Field of the Invention
The present invention relates to a suspension coil spring which is used in a suspension mechanism of a vehicle such as a car.
2. Description of the Related Art
The suspension mechanism of a vehicle such as a car comprises a suspension coil spring (hereinafter simply referred to as a coil spring) formed of a compression coil spring. Further, the suspension mechanism comprises a lower spring seat and an upper spring seat. The lower spring seat is disposed on the lower side of the coil spring. The upper spring seat is disposed on the upper side of the coil spring. The coil spring is compressed between the upper and lower spring seats by the weight (load) applied from above. Further, the coil spring extends and retracts in accordance with the magnitude of the load.
One of the causes of the breaking of the coil spring is the formation of rust as the coating of the coil spring is detached by thrown-up pebbles. If a corrosion pit is formed as the rust grows, the coil spring breaks from the corrosion pit. Hence, as disclosed in Patent Literature 1 (JP 2005-171297 A), it has been proposed to form a coating film of two-layer structure on the surface of a coil spring. An example of the coating film of two-layer structure is comprised of an epoxy resin based undercoat layer, and an epoxy polyester resin based topcoat layer formed on the undercoat layer.
An end turn portion of the coil spring includes a first portion, a second portion, and a third portion. The first portion is always in contact with the spring seat irrespective of the magnitude of the load. The second portion contacts the spring seat or is separated from the spring seat depending on the magnitude of the load. For this reason, in the second portion, foreign matter such as sand may stick between the end turn portion and the spring seat. The third portion is always separated from the spring seat irrespective of the magnitude of the load. Rust-inhibiting coating is applied to the surface of the coil spring. However, when the coil spring extends and retracts in a state where hard foreign matter such as sand is stuck between the end turn portion and the spring seat, the coating film detaches and rust is formed. Also, because of the sticking foreign matter, the surface of the coil spring may be damaged. When rust forms in the damaged part and the rust grows, it leads to the coil spring breaking.
As described in Patent Literature 2 (JPH08-200414 A), a suspension mechanism comprising means for preventing an end turn portion of a coil spring from wearing is publicly known. This suspension mechanism is provided with an insulator formed of a rubber block on a spring seat. This insulator supports the end turn portion of the coil spring. In a coil spring described in Patent Literature 3 (JP 2000-304079A), at an end turn portion of the coil spring, a tube made of synthetic resin is fitted. In the tube made of synthetic resin, a slit is formed along a longitudinal direction of a wire.
A coil spring of Patent Literature 1 has a coating film of two-layer structure which is constituted by an undercoat layer and a topcoat layer. This type of coil spring is effective against thrown-up pebbles. However, the coil spring has a little effect against corrosion or damage caused by sand, etc., trapped between an end turn portion and a spring seat. Moreover, the coating film of two-layer structure has the problem that the cost to be the incurred by coating is high as compared to a coil spring having a general coating film.
In the suspension mechanism of Patent Literature 2, on the lower spring seat, a block insulator is arranged. Since the insulator is quite heavy, the suspension mechanism has the problem that an unsprung weight becomes large. Moreover, foreign matter such as sand and dust may accumulate on an upper surface of the block insulator. When the foreign matter contacts the end turn portion, the wire is damaged.
In the coil spring of Patent Literature 3, the tube made of synthetic resin having a slit is fitted to the end turn portion. In this type of coil spring, when parts of the wire near the end turn portion come close to each other because of a large load such as in a full bump, the tube mentioned above is sandwiched between the parts of the wire and is compressed. Since the tube produces a repulsive force when this occurs, there is the fear that the load/deflection characteristics of the coil spring may be varied from the design value. Also, when the tube having the slit is fitted to the end turn portion, a corrosive material such as an antifreeze agent scattered on a road in winter enters into the slit. Such a corrosive material being adhered to the wire and remaining on the wire causes the wire to corrode.